The material to be separated may be settleable solids such as grit, sediments and fine particulates, or liquids having a lower density than the main liquid flow (which may be oils and hydrocarbons if the main liquid is water), or gross solids that may be neutrally buoyant, such as street debris including bottles, crisp packets, cigarette ends, leaves, etc.
Hydrodynamic treatment vessels or vortex separators are well known and are based on initial research work carried out in the 1950's and 1960's (Design, Construction and performance of vortex overflows, Bernard Smisson, Symposium on Storm Sewage Overflows, Institution of Civil Engineers, 1967, pages 99-110). They have found application as combined sewer overflows (CSOs) and as grit separators.
Separators known as “Hydro-Dynamic” separators are low energy devices which operate by causing a liquid containing suspended solid material to rotate in a cylindrical vessel so that the solid material falls under gravity and inertial forces to the base and once there is swept to a central lower outlet by an inward sweeping effect caused by complex flow patterns in the device. It is known that the device is suitable for providing enhanced settlement of solids material from a liquid/solid mixture. Thus, such devices have been used in sewage treatment for separating hard grit from the incoming raw sewage, with the resultant degritted sewage then being passed to a conventional sewage treatment plant. They are also used as “storm water overflows” upstream of conventional sewage treatment works to ensure that gross contamination is separated from liquid waste discharged to watercourses during storm conditions when the sewage treatment works is unable to cope with the high flow. “Hydro-Dynamic” separators of this type are described and claimed in, for instance, our British Patent Specifications Nos. 2082941 (corresponding to U.S. Pat. No. 4,451,366) and 2158741 (corresponding to U.S. Pat. No. 4,747,962).
The known hydro-dynamic separator is a simple device with no moving parts. The simple geometry of the device however, hides an internal complexity of flow structure. The mean flow pattern observed is a downward helical flow in the outer region and an upward helical flow near the central region of the separator. These two spiral flow regimes are separated by a shear zone region. The combination of underflow and overflow leads to a non-uniform axial flow profile. The effects of fluid viscosity, boundary layers and momentum transfer between adjacent zones of flow moving at different velocities, cause velocity gradients and vorticity (rotation) to be present. These result in a secondary flow, superimposed on the primary flow, which in turn results in solids being swept towards a lower outlet, and thence to a solids collection trough or hopper. The hydraulic regime in the separator ensures very little short-circuiting with a near plug-flow type flow regime.
The effectiveness of a hydrodynamic separator can be expressed quantitatively in terms of removal efficiency and retention efficiency. Removal efficiency represents the percentage of particles in the incoming flow which are removed from within the separator. Retention efficiency represents the percentage of particles or other contaminants in the incoming flow which are retained within the device and do not reach the clean water outlet. Both the removal efficiency and retention efficiency will vary according to the characteristics of particle concerned, such as its size, density and shape, but the objective is to maximise the percentage of particles either removed from the flow or retained within the treatment device, particularly for smaller particle sizes, such as those below 400 microns.
Optimising the performance of hydrodynamic separators is extremely difficult. Small changes in geometry can have major effects on removal and retention efficiencies, and consequently the optimisation process requires the construction of many prototypes. This is both expensive and time consuming, and does not lead to a guaranteed improvement in performance.
Computational fluid dynamics enables computer modelling of treatment vessels, and of the flow regimes which are created within them. However, the complex nature of the flow and, as mentioned above, the effect on performance of small changes in geometry mean that even computational fluid dynamics is not completely reliable in optimising the performance of hydrodynamic treatment devices in a practical environment.
A development of the separators disclosed in GB 2082941 and GB 2158741 is disclosed in WO00/62888. The separator disclosed in that document comprises a cylindrical vessel containing a hollow column which terminates at its lower end at a downwardly diverging conical member. Shortly above the conical member, the hollow column has openings which permit flow from the main body of the vessel into the column. There is an outlet for such flow at the top of the column.
A dip plate surrounds the column, and acts to stabilise flow patterns within the vessel. In operation, flow can take place around the dip plate to the openings in the column, and thence up the column to the outlet.
The present invention arises from a desire to increase the removal and retention efficiencies of separators of the type disclosed in GB 2082941, GB 2158741 and WO00/62888.